Honestly, the whole page count thing isn't the best metric for guessing reading time anyway. 'Harry Potter' books changed size and font drastically between UK and US editions, not to mention the switch to paperback. My old Scholastic hardcover of 'Order of the Phoenix' is like 870 pages, but the font is huge and the margins are wide; my Bloomsbury paperback of 'Goblet of Fire' is denser with smaller text even though the page count is lower.
What really affects my reading time is the content density. 'Chamber of Secrets' flies by because it's mostly straightforward adventure, but 'Half-Blood Prince' has so many memory-pensive scenes and layered dialogue that I read it slower, savoring the details. The later books are longer, but if you're invested, you might binge them faster than the shorter, earlier ones. For me, commitment came down to how gripped I was, not the raw page number.